System and method for injection moulding of plastic materials

ABSTRACT

A system and method for the injection moulding of plastic material through a plurality of injectors each having a pin valve displaceable in a controlled fashion between a closing position and an opening position by a respective actuator of an electronic control type, in which there is provided prompt activation of a preset function of changing a colour of the plastic material to be injected, actuated according to an optimized sequence which minimizes a number of moulding operations required to change the colour.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 15/007,722filed on Jan. 27, 2016, and claims priority to Italian PatentApplication No. TO2015A000059 filed on Jan. 28, 2015, the entiredisclosure of these applications is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention refers to the injection moulding of plasticmaterials into the cavity of a mould, and more in particular it regardsan injection moulding system including a dispenser of the plasticmaterial and a plurality of injectors each having a pin valvedisplaceable between a closing position and an opening position for theintroduction of plastic material under pressure into a cavity of themould.

STATE OF THE PRIOR ART

The displacements of the pin valve of each injector are carried outthrough a fluid or electrical actuator: in the second case, as describedand illustrated for example in document EP-2679374A1 on behalf of theApplicant, the electrical actuators are easily controllable with the aidof an electronic unit which operates according to process parameters forprecisely and accurately controlling both the position and the speed ofthe pin valve during the movements thereof from the closing position tothe opening position and vice versa. Such control is obtained bothaccording to set cycles for example according to data detected duringthe injection process and through specific algorithms. The electronicunit may be similarly combined with fluid (hydraulic and pneumatic)actuators.

In the operation of the moulding system there often arises the need forchanging the colour of the component to be moulded, thus necessarilyimplying preliminary operations for cleaning the plastic materialdispenser and injectors. This leads to the inevitable production shutdown and waste of a considerable amount of polymer: as a matter of fact,the cleaning, in the simplest form thereof, is carried out by settingthe process parameters regarding the new colour and moulding a givennumber of components, subsequently discarded, up to attaining thedesired aesthetic quality. It is thus fundamental to reduce the timesand discarded material related to the change of colour in the largescale production.

In addition, setting the cycle sequence for opening the injectors tocomplete the change of colour is conventionally evaluated at thediscretion of the operator and thus this is done manually by theoperator in question, hence implying additional production shut-downtimes.

STATE OF THE ART

Document “LOECHTER UMSPRITZEN”, Plastverarbeiter, Huething GmbH,Heidelberg, DE, Vol. 49, n^(o) 5, Jan. 5, 1998, pages 50-52 describes,as regards the change of colour of the plastic material, a cyclereferring to an injection moulding apparatus with three injectorsactuated in a cascade or sequential fashion, which simply provides forthe repetition of the normal sequential standard moulding cycleaccording to which the lateral injectors are opened first to fill thecorresponding regions of the mould cavity and then, after an empiricallyestablished time, the central injector is also opened. When the flow ofthe plastic material introduced into the mould by the central injectorreaches the two lateral regions, the two lateral injectors are closed.

Such repetition of the normal moulding sequential cycle would allow,according to this document, performing the colour change in 5-10moulding operations. This possibility, simply declared without anyexperimental evidence, obviously depends on a series of parametersregarding:

-   -   dimensions of the piece to be moulded, (length of the supply        channels and amount of the plastic material),    -   characteristics of the plastic material (viscosity, moulding        temperature),    -   injectors (number, position, type, control method)    -   colour of the plastic material.

This document does not specify any of these parameters: considering thefact that the described colour change cycles do not differ from thesequential injection in any manner whatsoever, it should be basicallydeemed that the indicated 5-10 moulding operations must be withreference to the most favourable conditions (small pieces to be moulded,high viscosity and temperature of the plastic material, limited numberof injectors, colour switch from yellow to white or vice versa).

As a matter of fact, it was basically proven that in case ofconsiderably less favourable conditions (large pieces to be moulded suchas for example vehicle panels or spoilers, low viscosity and temperatureof the plastic material, high number of injectors, colour switch fromblack to white) the number of moulding operations with sequentialstandards required for changing colour is markedly higher, even in theorder of several tens, with the ensuing long times and considerablewaste of plastic material.

The Applicant devised and protected, through a series of patentapplications yet to be published as of date of filing of the presentapplication, specific and particular methods for controlling the pinvalves of the injectors that allow, in particular in case of cascade orsequential injection, obtaining considerable improvement of the qualityof articles thus moulded. Typically in the sequential injection thestandard cycle provides for that some of the injectors, usually the mostperipheral ones with respect to the mould cavity, inject a amount ofplastic material lower than the one of the other injectors. Suchdifference in particular depends on the geometry of the dispenser andthe injectors, i.e. the section and length of the channels through whichthe molten plastic reaches the various injection points. The amount alsodepends on the opening degree of the respective pin valves and theoverall injection time.

SUMMARY OF THE INVENTION

The object of the present invention is to optimise the colour changeprocess, even automating it, so as to considerably reduce the timesrequired, and even significantly reduce the discarded materials andhence waste of plastic material.

The invention is based on acknowledging the fact that the control of theposition and displacement speed of the pin valve of each injector atevery instant, carried out through the activation of the relativeactuator (electrical or fluid actuator) by the electronic control unit,may be favourably exploited for accelerating the colour change process.

This taken into account, the invention provides an injection mouldingmethod as defined in the pre-characterising part of claim 1, whosedistinctive characteristic lies in the fact that it provides for theprompt activation of a preset function of changing the colour of theplastic material to be injected, actuated according to an optimisedsequence which consists in the complete opening of said some injectors(which inject a lower amount of material in a standard moulding cycle)to inject a maximum amount of plastic material for a limited number ofinjection cycles during which the other injectors (the ones that injecta higher amount of plastic material in a standard moulding cycle) remainclosed, thus minimising the number of moulding operations required forchanging the colour.

Thus, this distinctive method provides for allocating a higher amount tothe injectors that dispense less in the standard cycle, thus basicallyinverting the typical situation of the standard moulding cycle.

The preset colour change shall be processed as a function of the variousparameters obviously including the dimension and geometry of thearticles to moulded, as well as the characteristics of the material andthe number and position of the injectors provided for the mouldingthereof. Once the electronic control unit of the injection apparatus isprogrammed with the aforementioned preset function, the colour changecycle may be simply recalled and activated by the operator, thusconsiderably saving time and reducing waste of plastic material.

The invention is particularly advantageous in case of sequential orcascade injection apparatus for moulding large articles such as forexample vehicle body panels and the like in the automotive industry. Inthis case, the moulding apparatus comprises, in the known manner, firstand second injectors respectively arranged at a generally central areaand one or more generally peripheral areas of the mould cavity, and theinjection of the plastic material is carried out by sequentiallycontrolling the first and the second injectors so that the pin valves ofthe first injectors are displaced from the first closing position to theclosing position before the pin valves of the second injectors, so as toprogressively fill the mould cavity from the generally central areatowards the generally peripheral areas. In this case, the preset colourchange function according to the invention provides for maximising theamount of the plastic material through the complete opening of therelative pin valves while the first injectors remain closed. It has thusbeen proven by experiment that the number of moulding operationsrequired for completing the colour change operation and attaining thedesired aesthetic quality of the moulded articles is surprisinglyreduced and minimised, passing from a few tens up to several units.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail, purely by way ofnon-limiting example, with reference to the attached drawings, wherein:

FIG. 1 is partial schematic view showing a part of an injection mouldingsystem utilising the invention,

FIG. 2 is an elevational schematic view showing a configuration of themoulding system suitable for carrying out a sequential or cascadeinjection,

FIG. 2A is a front schematic view of the system of FIG. 2,

FIG. 3 is a diagram representing, as a function of time, the amountplastic material exiting from the injectors represented in FIG. 2, and

FIG. 4 is a table exemplifying the advantages of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows a part of a system for the injection mouldingof plastic materials comprising, in a generally conventional manner, aninjector 3 connected to a molten plastic material dispenser or hotchamber 2 and comprising a nozzle 5 provided at the free end thereofwith a nozzle terminal 6 in communication with the cavity of a mould Sthrough an injection gate. The flow of the plastic material through thenozzle terminal 6 is controlled by a pin valve 7 axially displaceablealong the nozzle 5, through an actuator 8, between a lowered closingposition, represented in FIG. 1, and a raised opening position.

The actuator 8 is, according to a preferred but non-limitingrepresentation of the invention, an electrical actuator, and more inparticular, a rotary electric motor: the arrangement thereof representedin FIG. 1 is provided purely by way of example, in that it can forexample be of the type described and illustrated in the previouslymentioned document EP-2679374A1 according to which the shaft of therotary electric motor 8 drives the pin valve 7 through a transmission ofthe reversible type typically including a screw and nut group as well asan oscillating lever.

The electric motor 8 is operatively connected to a programmable controlelectronic unit 4 to actuate the pin valve 7 in a controlled fashionaccording to set cycles, as a function of various perimeters. Inparticular, the electronic unit 4 is programmed for actuating the pinvalve 7 in a controlled fashion in terms of position and speed duringthe opening and closing of the injector 3.

FIG. 2 schematically represents a known arrangement of the mouldingsystem for obtaining large articles, such as for example vehicle panelsP, by means of a cascade or sequential injection. The system exemplifiedin FIG. 2 comprises a plurality of injectors respectively indicated with3 a, 3 b, 3 c and 3 d respectively positioned at a central area,intermediate area and terminal area of the mould cavity and actuated byrespective rotary electric motors 8 a, 8 b, 8 c and 8 d controlled bythe electronic unit 4.

FIG. 2A represents a front view of the moulding system of FIG. 2, fromwhich there can be observed the geometry of the panel P and thearrangement of the injectors 3 a-3 d respectively at a central area,intermediate area and terminal area of the mould cavity S. In addition,there can also be observed channels 10 of the dispenser 2, through whichthe molten plastic is conveyed from the extension of the press nozzle 9up to the single terminals 6 of the injectors 3. It is clear that thelength of the channels 10 is variable as a function of the position ofthe injectors 3 and thus also shall be the amount of the materialtraversing them.

The chart of FIG. 3 represents a possible driving cycle of the electricmotors 8 a-8 d regarding which the electronic unit 4 is programmed so asto perform the sequential injection. The diagrams indicated with A, B, Cand D represent, as a function of time, the amount respectively exitingfrom the central injector 3 a, the intermediate injectors 3 b and 3 cand the end injectors 3 d. As observable from the chart, which onlyexemplifies, as mentioned, one of the possible cycles that can beactuated by the electronic unit 4, in the stage of filling the mouldcavity the central injector 3 a (diagram A) is opened first and a givenamount, mainly depending on the degree of opening of the pin valve 7, isinjected into the cavity. Upon completing the filling of the cavityallocated thereto, the central injector 3 a starts closing up toreaching a zero amount value. The first intermediate injectors 3 bsimultaneously open up to reaching a maximum amount lower than themaximum amount reached by the central injector 3 a and for exampleequivalent to about half the amount thereof (diagram B). Similarly towhat has been illustrated above, upon completing the filling inproximity of the first intermediate injectors 3 a, the latter are closedand simultaneously the second intermediate injectors 3 c open, up toreaching maximum amount values always lower than the one initiallyreached by the central injector 3 a and for example similar to theamount of the first intermediate injectors 3 b (diagram C). Lastly, thecycle is similarly completed with the opening of the end injectors 3 d(diagram D). The amount of the plastic material introduced into themould cavity by the intermediate injectors 3 b, 3 c and the endinjectors 3 d is thus considerably lower than the one released by thecentral injector 3 a and it is obtained by mainly acting on the openingpositions of the single pin valves 7.

Given that the moulding system described above requires modifying thecolour of the component to be moulded, a process for cleaning thedispenser 2 and the injectors 3 a-3 d should be carried out. Thisrequires the moulding of a given number of components with the plasticmaterial of the new colour, which are discarded until the requiredaesthetic quality is attained.

Thus, according to the distinctive characteristic of the invention, theelectronic control unit 4 is designed, i.e. programmed, for the promptactivation of a preset colour change function, actuated according to anoptimised sequence which minimises the required number of mouldingoperations. Such preset function is preferably determined with the aidof finite elements analysis and, in case of the example of sequentialinjection described herein, it provides for a limited series ofinjection cycles actuated with standard process parameters correspondingto those illustrated in the chart of FIG. 3, followed by a limitednumber of cycles in which the injectors characterised by the loweramount of the plastic material during the standard injection process, orthe injectors 3 b, 3 c and 3 d in this case, are completely openedmoving the relative pin valves 7 to the maximum or total openingposition while the injector 3 a is kept closed.

As represented in the table of FIG. 4, this sequence allows completingthe colour change, for example from black to a light colour, within only11 moulding operations 7 of which are obtained with the standard processparameters and 4 with total opening of the injectors 3 b, 3 c, 3 dkeeping the injector 3 a closed.

As observable from the table, this sequence—indicated with “4”—isconsiderably lower than the conventional sequence indicated with “1”, inwhich there are typically provided for at least 32 moulding operationswith the standard process parameters, and even considerably lower thanthe case where all injectors 3 a-3 d are completely opened for severalmoulding operations (sequence “2”) for colour change and the case wherea given number of moulding operations are carried out with the standardprocess parameters followed by other moulding operations with all theinjectors 3 a-3 d open (sequence “3”).

The method according to the invention obviously allows minimising boththe time required to change colour and the discarded pieces until therequired aesthetic quality is attained with the relative reduction ofwaste of plastic material.

The electronic control unit 4 may be programmed with various presetcolour change functions, for example according to the number ofinjectors and dimensions of the components to be printed, which may bepromptly recalled and activated automatically without requiring furtherinterventions by the operator, up to completion thereof.

The preferred but non-limiting use of electrical actuators has thefurther advantage of being able to accelerate the removal of residualmaterial from the terminal area 6, which is particularly critical in thecolour change process. As a matter of fact, the injector is opened to aminimum level, so that the injection pressure in proximity of the lowerend of the pin valve 7 is extremely high and the removal of the residualmaterial is thus easier.

Obviously, the construction details and the embodiments of the mouldingsystem may widely vary with respect to what has been described andillustrated, without departing from the scope of protection of thepresent invention as described in the claims that follow.

1. A system for the injection molding of plastic material through aplurality of injectors each having a valve pin displaceable in acontrolled fashion between a closing position and an opening position bya respective electronically controlled actuator according to set cycles,the system comprising: said set cycles including a standard injectioncycle wherein some of the plurality of injectors inject an amount ofplastic material lower than a maximum amount and lower than that ofother of the plurality of injectors which inject a lesser amount ofmaterial as compared to the some of the plurality of injectors; anelectronic control unit configured to activate a preset function ofchanging a color of the plastic material to be injected, actuatedaccording to a color changing sequence different from the standardinjection cycle, the color changing sequence including a completeopening of said some of the plurality of injectors to inject a maximumamount of plastic material for a limited number of injection cyclesduring which the other of the plurality of injectors remain closed, soas to reduce a number of molding operations required to change the coloras compared to using the standard injection cycle to change the color ofthe plastic material to be injected.
 2. The system according to claim 1,wherein the actuators are electrical actuators.